It is well known that almost every solid substance has a substantial, positive coefficient of thermal expansion, i.e., it expands significantly when heated. Up to the present time three families of crystalline ceramic materials have been found useful in the technology of low expanding ceramics: eucryptite-spodumene (LiAlSiO.sub.4 -LiAlSi.sub.2 O.sub.6), the main constituent of cooking ware; cordierite (Mg.sub.2 Al.sub.4 Si.sub.5 O.sub.18), the main component of steatite bodies; and zircon (ZrSiO.sub.4), well known for use in refractories. However, most of these known ceramics still have some positive coefficient of thermal expansion.
Components of earth orbiting satellites and instruments used on such satellites are subjected to wide and sometimes sudden changes in temperature in outer space. This occurs as the satellite travels in and out of the direct sunlight and the earth's shadow, or while the satellite itself is spinning, or perhaps when a temperature control system malfunctions. Due to thermal expansion and contraction the sudden changes in temperatures can cause a component to distort or fracture resulting in its failure to carry out its function. This problem is also acute for optical systems in the space environment, specifically for a reflecting film carried on a substrate, where any change can distort an image.
Some applications for mirrors operating even at room temperature are susceptible to distortions from even minute changes in temperature. For example, the demand for increased capacity of integrated circuit chips is pushing the capabilities of microphotolithographic projection devices that depend on mirror systems of extremely high precision.
Even electron beam microlithography instruments--which do not employ mirrors or other optical elements in the ordinary sense--have reached a stage of perfection where dimensional variation in structural components, e.g., the frame, due to even minute changes in ambient temperature, are a factor to be considered in its adverse impact on resolution capability of the apparatus.
One well-known approach to minimizing the problem with composite structures is to fabricate the composite with components having matching thermal expansion characteristics. However, this approach has been found not to succeed as well as desired in the case of thin films on supporting substrates.
Ceramics of the type NaZr.sub.2 P.sub.3 O.sub.12 have been utilized for electrical application involving ionic conductivity. Substitutions of elements are typically made to enhance this conductivity. For example, additional sodium has been substituted for some of the zirconium in a class of compounds, Na.sub.1+4z Zr.sub.2-z P.sub.3 O.sub.12.
Similarly, silicon and sodium have been substituted jointly for a portion of the phosphorous to create a solid electrolyte which is used in batteries. Such silicon-containing compounds are described as Na.sub.1+x Si.sub.x Zr.sub.2 P.sub.3-x O.sub.12. J. P. Boilot and J. P. Salantie, as reported in "Phase Transformation in Na.sub.1+x Si.sub.x Zr.sub.2 P.sub.3-x O.sub.12 Compounds", Material Research Bulletin Vol. 14, pp. 1469-1477, 1979, studied phase transformation in the latter class of compounds, and compared this feature with thermal expansion for the range of compositions corresponding to x ranging from 3 down to 1. As x was decreased from 3 to 2 the thermal expansion coefficient was reported by Boilot, et al. to change inversely, viz., to increase to more than twice its value. At x=1 there was a substantial reversal to a negative thermal expansion coefficient. These changes, in this scientific study, were correlated to changes in crystal structure. In particular, there is a reversible change in crystal structure when the material is heated. Although Boilot mentioned that the compound where x=1 displays an important shrinkage which could allow this material to be used when expansion is undesirable, he also recognized and pointed out the deleterious effect of a crystal structure that transforms upon heating.
Such effects are well known and generally avoided. For example, simple zirconium oxide (ZrO.sub.2) changes its crystal structure upon heating and cooling, and fractures itself in the process. Thus zirconium oxide, which is used as a refractory in high temperature barrier applications, is virtually always stabilized by the addition of other elements to prevent the crystal change and the corresponding self-destruction that may occur during extreme thermal changes. Generally those skilled in the art will avoid using any ceramic that changes its crystal structure during temperature changes.
Studies on similar solid electrolytes were reported in H. Y. P. Hong, "Crystal Structures and Crystal Chemistry in the System Na.sub.1+x Zr.sub.2 Si.sub.x P.sub.3-x O.sub.12 ", Material Research Bulletin, Vol. 11, pp. 173-182, 1976. Values of x from 0 to 3 are included, as are several compounds in the z-series, but no information is provided on thermal expansion.
In view of the foregoing, a primary object of the present invention is to provide an improved process for producing materials having very low or nearly zero coefficient of thermal expansion.
Another object of the present invention is to provide a novel structural component having extremely high dimensional stability which does not distort or fracture in a temperature changing environment.
A further object of the present invention is to provide a novel ceramic substrate which supports an optically reflecting film, has nearly zero thermal expansion coefficient, and does not fracture from changes in crystal structure.
A still further object of this invention is to provide an improved optically reflecting component useful for precision telescopic and microlithography projection system.